1. Field of the Invention
The invention relates to a spatio-temporal filter unit, provided with a sigma filter, to reduce the noise in images.
The invention further relates to an image display apparatus comprising such a spatio-temporal filter unit.
2. Description of the Related Art
A unit of the kind described in the opening paragraph is known from G. de Haan et al, “Memory integrated noise reduction for television,” in IEEE Transactions on Consumer Electronics, Vol. 42, No 2, May 1996, Pages 175–181. In this article, a unit is described which is a cascaded system of two filters, i.e., a spatial filter and a temporal filter. The spatial and temporal filters together should result in effective noise reduction. In general, spatial filters require pixels from one image, while temporal filters require samples from two or more successive images.
The spatial filter is a recursive sigma filter. It is based on a variant of the sigma nearest neighbors selection process. The combination process involves weighted averaging, which is optimal for additive white Gaussian noise. The weights of the described sigma filter can have three possible values, which are selected based on thresholds that are adapted to the noise level. In the article, also a noise estimator is described to estimate the noise level.
The temporal filter is realized as a motion-compensated first-order temporal recursive filter. Therefore, this temporal filter requires a motion estimation and motion compensation system, which makes the concept expensive. Furthermore, a motion detection means is necessary to adapt the strength of the filtering when motion occurs. Otherwise, the image is distorted.